The operating environment within a gas turbine engine is both thermally and chemically hostile. Significant advances in high temperature alloys have been achieved through the formulation of iron, nickel and cobalt-base superalloys, though components formed from such alloys often cannot withstand long service exposures if located in certain sections of a gas turbine engine, such as the turbine, combustor or augmentor. A common solution is to protect the surfaces of such components with an environmental coating system, such as an aluminide coating or a thermal barrier coating (TBC) system. The latter typically includes an environmentally-resistant bond coat and a thermal barrier coating of ceramic deposited on the bond coat. Bond coats are typically formed from an oxidation-resistant alloy such as MCrAlY where M is iron, cobalt and/or nickel, or from a diffusion aluminide or platinum aluminide that forms an oxidation-resistant intermetallic. During high temperature excursions, these bond coats form an oxide layer or scale that bonds the ceramic layer to the bond coat. Zirconia (ZrO.sub.2) that is partially or fully stabilized by yttria (Y.sub.2 O.sub.3), magnesia (MgO) or other oxides has been widely employed as the material for the ceramic layer. The ceramic layer is typically deposited by air plasma spraying (APS), low pressure plasma spraying (LPPS), or a physical vapor deposition (PVD) technique, such as electron beam physical vapor deposition (EBPVD) which yields a strain-tolerant columnar grain structure.
While thermal barrier coating systems provide significant thermal protection to the underlying component substrate, internal cooling of components such as turbine blades (buckets) and nozzles (vanes) is generally necessary, and may be employed in combination with or in lieu of a thermal barrier coating. Airfoils of turbine blades and nozzles of a gas turbine engine often require a complex cooling scheme in which cooling air flows through the airfoil and is then discharged through carefully configured cooling holes. For example, diffuser cooling holes require noncircular openings at the surface of the airfoil in order to distribute the cooling film over the airfoil contour and therefore increase the effectiveness of the cooling film. The performance of a turbine airfoil is directly related to the ability to provide uniform cooling of its surfaces with a limited amount of cooling air. Consequently, processes by which cooling holes and their openings are formed and configured are often critical because the size and shape of each opening determine the amount of air flow exiting the opening and the distribution of the air flow across the surface of the airfoil, and affect the overall flow distribution within the cooling circuit containing the opening. Other factors, such as backflow margin, are also affected by variations in opening size.
For airfoils without a thermal barrier coating, cooling holes are typically formed by such conventional drilling techniques as electrical-discharge machining (EDM) and laser machining. Complex advanced casting practices have also been used to yield airfoil castings with dimensionally correct openings. However, EDM cannot be used to form cooling holes in an airfoil having a ceramic TBC since the ceramic is electrically nonconducting, and laser machining is prone to spalling the brittle ceramic TBC by cracking the interface between the airfoil substrate and the ceramic. Accordingly, cooling holes have been required to be cast or formed by EDM and laser machining prior to applying the TBC system, limiting the thickness of the TBC which can be applied or necessitating a final operation to remove ceramic from the cooling holes in order to reestablish the desired size and shape of the openings.
From the above, it can be seen that manufacturing an air-cooled airfoil protected by a TBC is complicated by the requirement that the cooling holes remain appropriately sized and shaped in order for the cooling film produced by the holes to uniformly cool the external surfaces of the airfoil. The service life of an air-cooled airfoil that has been coated with an insulating ceramic layer is detrimentally affected if the ceramic layer alters the shape or reduces the size of the cooling hole openings from that required by the airfoil design. Accordingly, a method is desired for producing an air-cooled airfoil that is protected by a TBC system yet has appropriately sized and shaped cooling holes and openings, and particularly noncircular-shaped diffuser-type openings that promote a uniform distribution of cooling film across the external surface of an airfoil.